Quote

Any two-argument open(), readpipe() (aka qx//) and similar
operators found within the lexical scope of this pragma will
use the declared defaults. Three-argument opens are not
affected by this pragma since there you (can) explicitly
specify the layers and are supposed to know what you are
doing.

Quote

Use 3-argument form to open a file with arbitrary
weird characters in it,

open(FOO, '<', $file);

otherwise it's necessary to protect any leading and
trailing whitespace:

$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");

(this may not work on some bizarre filesystems).
One should conscientiously choose between the magic
and 3-arguments form of open():